MICRODOSIMETRY SPECTRA OF THE LOMA LINDA PROTON-BEAM AND RELATIVE BIOLOGICAL EFFECTIVENESS COMPARISONS

Protons have long been recognized as low LET radiation in radiotherapy . However, a detailed account of LET (linear energy transfer) and RBE (relative biological effectiveness) changes with incident beam energy and depth in tissue is still unresolved. This issue is particularly im portant for treatment planning, where the physical dose prescription i s calculated from a RBE using cobalt as the reference radiation. Any s ignificant RBE changes with energy or depth will be important to incor porate in treatment planning. In this paper we present microdosimetry spectra for the proton beam at various energies and depths and compare the results to cell survival studies performed at Loma Linda. An empi rically determined biological weighting function that depends on linea l energy is used to correlate the microdosimetry spectra with cell sur vival data. We conclude that the variations in measured RBE with beam energy and depth are small until the distal edge of the beam is reache d. On the distal edge, protons achieve stopping powers as high as 100 keV/mu m, which is reflected in the lineal energy spectra taken there. Lineal energy spectra 5 cm beyond the distal edge of the Bragg peak a lso show a high LET component but at a dose rate 600 times smaller tha n observed inside the proton held. (C) 1997 American Association of Ph ysicists in Medicine.